>>I do not understand this. Did they count the rings of a nearly 4,000-year-old tree that was still growing at the bottom of the lake?<<
No. Where in the article is a “still growing” tree mentioned? Rather, the scientists compared the rings still identifiable in cross-sections of fragments of the DEAD wood belonging to the find with established dendrochronological records relevant to that geographical region.
http://en.wikipedia.org/wiki/Dendrochronology
At the risk of being obvious, nowhere.
That being the case, I must have meant something else, mustn't I?
That "something else" was to point out that the idea was patently absurd.
Simply counting the rings in a piece of dead log provides no useful data, in the absence of context. What was missing from the article (and also missing from my understanding) was the context.
Thank you for the link.